This invention relates to communication systems and more specifically to sonar type underwater communication systems using digital codes.
Underwater communication systems using acoustic signals are known in the art. In such systems, intelligence is conveyed by means of a modulated acoustic wave propagating between immersed transmitting and receiving transducers.
The reliability of such systems, however, is limited since various portions of a given signal may travel along diverse paths before reaching the receiver. Thus, a portion of a given signal may pass directly between the transmitter and the receiver whereas other portions may first be reflected from intervening obstacles or refracted because of variable density in the transmission medium.
Because the signal energy travels over diverse signal paths, a short burst of sound energy originating at the transmitter may be received as a group of separate bursts, each having a different transmission delay. Moreover, each path also includes a number of subpaths. Because of this, the transmission medium causes selective fading. Furthermore, bursts arriving over different paths may have different amplitudes.